1. Field of the Invention
The present invention relates to offshore drilling vessels. More particularly, the present invention relates to underwater fairleads. Even more particularly, the present invention relates to underwater fairleads with removable pocket segments.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98.
An offshore drilling vessel is essentially a massive, floating, mobile vessel used in the offshore exploration and/or production of oil and gas. The vessel is equipped with the necessary drilling tools to drill an oil and gas well into the ocean floor or produce the oil and gas to the surface for recovery. Until recently, the typical offshore drilling vessel was of the type that generally includes two large pontoon hulls, at least four vertical support columns, and a drilling platform. The pontoon hulls float in the water horizontally and are parallel to one another. At least two support columns are attached to and extend vertically upwardly from each pontoon hull. The drilling platform is horizontally attached upon the vertical columns. This type of vessel is sometimes referred to as a “semi-submersible” vessel, and is towed from drilling site to drilling site by one or more boats.
Recently, another type of offshore drilling and production vessel has been introduced to the market; this type of vessel is sometimes referred to as a spar-type vessel. This type of vessel is similar to the older semi-submersible type vessel in that it includes a drilling and/or production platform, but differs in the flotation mechanism upon which the platform rests. More particularly, instead of using pontoon hulls and four vertical supports to support the platform, the spar-type vessel supports the platform with a single, long, slender cylinder, or support column that is vertically moored in the water. This support column is sometimes referred to as a Deep Draft Caisson Vessel, or DDCV. As just one example, the cylinder may have a diameter of approximately 120 feet, and a length of approximately 500 feet. In this example, when moored in the drilling and/or production position, there may be approximately 50 to 80 feet of the cylinder exposed above the water line, with the remainder disposed below the water line.
Irrespective of the type of vessel, whether it be the older “semi-submersible” type or the newer “spar” type, the vessel must be anchored to the ocean floor before drilling begins. With a semi-submersible vessel, which is generally rectangular in shape, there will be at least one large mooring line, and sometimes more than one mooring line, at each corner of the vessel. Each mooring line is in turn connected to a large anchor at the ocean floor. With the spar-type vessel, there will be a number of anchors, perhaps as many as twelve, attached about the circumference of the support column, or DDCV, upon which the platform rests. With both types of vessels, each mooring line is tensioned or relaxed by its own mooring unit, through the use of a wire rope, chain, or combination wire rope/chain mooring line. In very broad terms, a mooring unit is essentially a giant hoist. Each type of vessel is also equipped with a “fairlead” for each mooring unit. A fairlead is essentially a pulley or sheave. The fairleads are mounted to the vessel directly below each mooring unit. With the semi-submersible vessels, the fairleads are mounted to the vertical support members. With the spar-type vessels, the fairleads are mounted directly to the vertical support column, or DDCV. For each mooring unit, the mooring line exits the mooring unit and passes around its corresponding fairlead pulley or sheave before being connected to the anchor.
Various patents have issued in the past relating to underwater fairleads. For example, U.S. Pat. No. 7,240,633 issued on Jul. 10, 2007, to Barlow, describes an underwater chain stopper and fairlead apparatus for anchoring offshore structures. The apparatus has a mounting member, a fairlead member and a chain stopper member. The mounting member is attached to an underwater surface of the offshore structure or vessel and includes a bracket for coupling the fairlead member. The bracket may have a hinge allowing the fairlead member to pivot in an approximately horizontal plane. The chain stopper member is coupled to the fairlead member through a hinge which allows the chain stopper member to pivot with respect to the fairlead member in an approximately vertical plane. The chain stopper member includes a chain stopper flapper having a horseshoe shaped opening at one end. The other end of the flapper is connected to the chain stopper member through a hinge which allows the flapper to swing between an open position and a closed position. In the open position, the chain links for the anchor chain are allowed to pass by the horseshoe shaped opening on the flapper. The horseshoe shaped opening also includes a chain link seat which stops movement of the anchor chain through the chain stopper member when the flapper is in the closed position. The chain stopper flapper moves to the closed position under the force of gravity to provide a self-locking chain stopper. A latch mechanism is provided for latching the chain stopper flapper in an open position.
U.S. Pat. No. 6,148,755 issued on Nov. 21, 2000, to Wudtke et al., describes a removable underwater fairlead. The fairlead is capable of being remotely uninstalled from and then reinstalled on an offshore drilling and production vessel, while the vessel is in its moored position, in the event that the fairlead develops mechanical difficulties that require repair. The fairlead may be remotely uninstalled and reinstalled while the fairlead is underwater and without ballasting the vessel far enough to raise the fairlead above the water surface. This is useful in fairleads that are mounted to a vessel of the type where the ballasting process can be a very difficult, time-consuming, and expensive process.
U.S. Pat. No. 6,817,595 issued on Nov. 16, 2004, to Jenkins et al., describes a swing arm type chain support. The swing arm type chain support has a hook arrangement integral with a swing arm which allows a mooring chain to ratchet over the hook while tensioning, then automatically engages the hook when tension is reduced.
U.S. Pat. No. 4,742,993 issued on May 10, 1988, to Montgomery et al., describes a self-aligning quadrant fairlead. The fairlead is for mooring lines and has an arcuate row of grooved line-engaging shoes of plastic material which shoes can be removed and replacement shoes can be installed without disassembling the mooring line from the fairlead. The fairlead is of generally triangular shape and is supported by trunnion and bearing means at its upper tip and adjacent to its apex enabling the fairlead to swing about an upright axis for self-alignment with a stretch of mooring cable beyond the lower tip of the fairlead.
U.S. Pat. No. 7,104,214 issued on Sep. 12, 2006, to Niebur, teaches a fairlead with an integrated chain stopper. The fairlead is for guiding and securing an anchor chain between an offshore structure and an anchor. The fairlead comprises a fairlead frame, a pivoting latch, and an actuator. The fairlead frame is pivotally mounted to the offshore structure and supports an axle for a chain sheave. The pivoting latch is mounted to pivot on the axle and comprises a tension link with a chain latch and a counterweight for urging the chain latch into engagement with the chain. The pivoting latch is configured to engage the chain only when the chain is traveling in the payout direction. The actuator controls the action of the counterweight.
U.S. Pat. No. 4,070,981 issued on Jan. 31, 1978, to Guinn et al., teaches a mooring system for floating drilling vessels. The mooring system absorbs all of the forces on the vessel, such as wind, current, wave, swell, roll, pitch, heave, surge and sway. These forces are measured by sensing load on a motor, electric, hydraulic, and the like, driving the anchor chain wildcat while hauling it in, by sensing load on the brake bands for the windlass wildcats, and by sensing load on the chain stopper, which sensed loads are transmitted to a display device which provides sufficient information to maintain the drilling vessel within the alignment limits and to warp it into the sea to minimize forces and motions of the vessel and to avoid beam sea forces. A chain counter is provided on the power wildcat which counts the links, and hence the distance, the anchor chain is payed out or hauled in, which is transmitted to the display device. The anchor chains extend from each side both fore and aft of the vessel and each anchor chain has an electric motor driven wildcat, and extends through a chain stopper and fairlead. Vessel alignment is displayed on a cathode ray tube using an acoustic position resonance system.
U.S. Pat. No. 4,311,109 issued on Jan. 19, 1982, to Webb, describes a fairlead. The rotatable fairlead is suitable for use in a deployment, control and recovery of anti-pollution booms, and has two end pieces, two pins, or rollers, which are mounted between the end pieces and spaced sufficiently apart such that a partially inflated boom may pass between the pins. Means are provided for rotating the end pieces and pins through at least 270°. Means for holding them at a given angle of rotation are also provided.
U.S. Pat. No. 5,845,893 issued on Dec. 8, 1998, to Groves teaches an underwater self-aligning fairlead latch for mooring a structure at sea. The fairlead latch device is used for guiding and securing an anchor chain between an offshore structure and an anchor. The fairlead device includes a latch housing pivotally mounted to a fairlead housing. The latch housing includes one or more latches for securing the anchor chain in place. The fairlead housing includes a bending shoe which guides the anchor chain from its orientation within the bending shoe up the platform column to the deck. The fairlead housing is pivotally mounted to the offshore structure.
It is an object of the present invention to provide an underwater fairlead assembly which improves lead times in production and replacement.
It is another object of the present invention to provide an underwater fairlead assembly which reduces the risk of defects in the cast components of the fairlead assembly.
It is yet another object of the present invention to provide an underwater fairlead assembly which has a reduced weight when compared to fairlead assemblies of the prior art.
It is a further object of the present invention to provide an underwater fairlead assembly whose geometry reduces chain link bending fatigue stresses.
It is yet another object of the present invention to provide an underwater fairlead assembly whose geometry reduces chain link bearing stresses.
It is another object of the present invention to provide an underwater fairlead assembly which reduces pocket bearing stresses.
It is another object of the present invention to provide an underwater fairlead assembly which makes retrieval, replacement, or repair of the casting or bearings simpler.
These and other objects and advantages of the present invention will become apparent from a reading of the attached specification.